The present invention relates to an error correction apparatus, or more in particular to a code error correction apparatus highly capable of error correction even in case of generation of many code errors of a digital information signal.
A conventional code error correction apparatus, as described in JP-A-57-10561, encodes a transmission data in duplex way to form a product code block including an inner code block and an outer code block. After the inner code block is decoded, a pointer is added to each word in each block for which an error is incapable of being corrected. At the time of decoding the product code block using the outer code block, the status of the particular pointer is checked thereby to improve the error correction ability while at the same time preventing error detection failure or erroneous correction. In a method for further improving the error correction ability of this code error correction apparatus, as disclosed in JP-A-63-211923, the same signal block is transmitted a plurality of times, and a data determined as an error in the first received signal is replaced by a second or subsequent corresponding data in order to reduce data errors.
The technique disclosed in JP-A-63-211923 is effective as long as the amount of error is comparatively small and at least one of a plurality of times of data transmission is correct. Nevertheless, a system to which this conventional technique is required to be applied has an extremely great number of data errors in most cases. In such a system, all the blocks may contain an uncorrectable error, or even when an error is determined correctable, may have so many a data error as to defy a decision. Even against such a transmission signal, it is necessary to form a product code block by combining signals as apparently correct as possible for decoding an outer code. In an audio data for digital VTR having a D2 format, for example, the first block data is recorded in an area near to the trailing end of magnetic tape and the second block data in an area near to the leading end of the next track. Taking the tracking stability of a reproducing head into consideration, the data at the trailing end of track, that is, the first data is more apparently correct. In the aforementioned conventional system, however, if the first data is determined erroneous, the second data is undesirably employed even if erroneous. As a result, the more apparently correct data is thus erased, thereby posing the problem that a signal which otherwise could have been corrected with an outer code with the first data becomes uncorrectable.
Another method in which the error correction ability is further improved is disclosed in JP-A-63-317990. According to this method, a product code block recorded in a medium is reproduced a plurality of times or a product code is transmitted a plurality of times, so that a data given by a code error correction apparatus is decoded by an inner code (first check word) and then by an outer code (second check word). While the data is being decoded by an outer code, a data with as small errors as possible is selected out of those data reproduced a plurality of times and corrected by error detection with an inner code thereby to improve the correction ability. In this method, even when an error could not be detected by an inner code, a plurality of reproduced data are compared with each other to detect an error for an improved data reliability.
The technique disclosed in JP-A-63-317990 is such that after the product code blocks reproduced a plurality of times are stored in memory respectively, the data and error flag are read out of each memory at the same time. These data are compared with each other, while selecting a data as free of an error uncorrectable by inner code as possible and decoding it by outer code. Further, the data are compared to check for error detection failure. This method, however, fails to take into consideration the required memory capacity. If the block made up of product codes is large in size or the same block is subjected to recording and reproduction a number of times, therefore, a number of memories of large capacity are required.